1. Field of the Invention
The present invention relates generally to moisture detectors, and more specifically to a moisture content detector for a hermetically sealed semiconductor device.
2. Description of the Prior Art
Moisture within hermetic packages may cause premature device failure due to electrogalvanic corrosion of chip metallization. Knowledge of package moisture contents thus becomes a critical parameter both for operating reliability as well as process technology improvements and quality control.
Mass spectroscopy has been the definitive method for measuring package moisture. It is relatively expensive to either acquire the instrumentation or for per-sample charges by service laboratories. The test is destructive to the sample package and if not executed carefully, it can also be destructive to the encapsulated device precluding further failure analysis. Results are not obtainable at the time that the need for them arises.
An in-situ monitor, in which a sensing device is mounted directly in a sample package, or incorporated into the design of a circuit, offers rapid availability of moisture data. Analysis cost per sample is inexpensive, only about 10% of the cost of a mass spectrometer analysis. This means that statistically significant numbers of packages can be analyzed on a more frequent basis. The in-situ monitor thus enables more exacting process development experimentation and process control measurements, and speeds up vendor and assembly lot qualifications. Moreover, the in-cavity sensor gives a real-time dynamic analysis of moisture within a package. The determination can be made repeatedly so that moisture conditions can be monitored as a function of part storage or operating lifetime.
Two classes of in-situ sensors for integrated circuit packages have been developed. The first is the surface conductivity sensors and the second is the volume effect sensors. The surface conductivity sensors are those which measure the conductivity of moisture condensed on a non-porous surface between two spaced electrodes using a "dew point test". The volume effect sensor measures the conductivity of a porous surface which absorbs moisture. In an experiment, reported by D. E. Meyer in "Miniature Moisture Sensors For In Package Use By The Microelectronics Industry", Reliability Physics Symposium, Las Vegas, Nevada April 1975, an Al.sub.2 O.sub.3 volume effect sensor was tested with a fired thick film interdigitated surface conductivity sensor and an interdigitated thin aluminum film on oxidized silicon dew point-resistivity sensor. The thick film sensor did not provide sensitivity below 10,000 ppmv. The dew point-resistance sensor did not provide repeatable results, which was attributed to the moisture selectively condensing on the package surface instead of on the sensor. The study concludes that A1.sub.2 O.sub.3 volume effect sensors are far superior, although they cannot withstand elevated package sealing temperatures. Thus these two surfaces conductivity sensors were abandoned and industry directed their efforts to the volume effect sensors.
There are many examples of the volume effect sensors used as humidity sensors in hermetically sealed integrated circuit packages. U.S. Pat. Nos. 3,943,557 and 4,050,048 to Frazee discuss the use of cobalt oxide as a hygroscopic media formed on a surface of the package to detect the percentage of humidity in the package.
The formation of a moisture sensor on an integrated circuit wherein selected portions of the silicon chip are made porous by an anodic etching and a pair of aluminum conductors is formed above the porous area as described in U.S. Pat. No. 4,057,823 to Burkhardt, et al. In each of these patents, the relative humidity is determined by measuring changes in the resistance or the capacitance of the porous structure when water molecules diffuse therein.
These devices illustrated in these patents require special material, for example, cobalt oxide or other moisture sensitive oxide complexes or may include additional steps for preparing a hygroscopic surface. Similarly, almost all these detectors measure the change in resistance or capacitance of the hygroscopic surface to indicate relative humidity. While such sensors are useful for low-temperature sealed packages, their fragile A1.sub.2 O.sub.3 structure does not withstand elevated temperatures and they are not suitable for solder glass sealed ceramic packages.
Another type of moisture detector was presented by R. P. Merrett and S. P. Sim at the ARPA/NBS workshop, National Bureau of Standards, Mar. 22-23, 1978 in a talk entitled "Assessment Of The Use Of Measurement Of Surface Conductivity As A Means Of Determining Moisture Contents Of Hermetic Semiconductor Encapsulations". Although the conductors were formed on a nonporous SiO.sub.2 surface, the measurement of the SiO.sub.2 surface conductivity was utilized in a "dewpoint test" by using the field effect structure to detect the changing of the surface of the oxide surrounding a biased electrode.
Although knowledge of the relative humidity of a hermetically sealed package or a humid environment may be desirable, the prior art has not directed itself to detecting the moisture content in parts per million by volume of hermetically sealed semiconductor package or other sealed environments. In light of the new government specification requirements for limitation of moisture contents of hermetic devices, to below 10,000 ppm per volume namely MIL-STD-883B, method 1018, an in-situ sensor is desirable for determining the water content of hermetically sealed devices to below 10,000 ppm by volume.